1. Field of the Invention
The present invention relates generally to a method device for leveling a series of metal strip to subject heat treatment, such as annealing. More specifically, the invention relates to a technique for effectively performing metal strip leveling operation with avoidance of breakage at welded joint, or defects in the series of metal strip.
2. Description of the Background Art
In general, a plurality of hearth rolls are arranged in a continuous annealing furnace for defining a zig-zag path through known heat treatment stages, such as a heating stage, a soaking stage maintaining the metal strip at constant heated temperature, a cooling stage and so forth. The metal strip is wound around the hearth rolls and runs through the zig-zag path in the annealing furnace to be subjected to heat treatment.
In the practical operation of the annealing furnace, the metal strip at the zone cross to the inlet has a relatively low temperature. At this zone in the annealing furnace, the temperature at the transverse edge positions of the hearth roll tends to become much higher than that at the transverse central portion. This temperature difference between the edge portions and the central portion increases with increase of line speed. This causes a substantial difference of heat expansion at the edge portion and the central portion to cause reduction of the effective crown magnitude. This reduction of the crown tends to lead to unbalance of tension to be exerted on the metal strip in the transverse direction in the case that the metal strip is in the configuration of edge wave or center wave. Unbalanced tension tends to cause meander of the metal strip in the path.
In order to prevent this, the line speed to feed the metal strip through the annealing furnace has to be lowered in order to minimize the temperature difference between the edge portions and the central portions of the hearth roll. This clearly causes lowering of the efficiency of the annealing furnace. On the other hand, possibility of meander of the metal strip can be reduced by providing greater magnitude of the initial crown on the periphery of the hearth roll. This may allow maintaining a satisfactory line speed. On the other hand, when sufficient initial crown for the initial heating stage is provided for the hearth roll, the crown magnitude tends to be excessive at the high temperature zone to cause heat buckling when the line speed is lowered for some other reason, such as for welding the strips.
In this view, there are various proposals for obtaining stable operation of the annealing furnace by controlling magnitude of crown on the peripheries of hearth rolls. For example, such proposals have been disclosed in the Japanese Utility Model First (unexamined) Publication (Tokkai) Showa 57-177930 and the Japanese Utility Model First Publication (Jikkai) Showa 58-10546. In these prior proposals, crown magnitudes have been controlled by heating and cooling the hearth rolls. On the other hand, the Japanese Patent First Publication (Jikkai) Showa 55-172359 proposed to control the crown magnitude by means of a bending device.
On the other hand, the meander of the metal strip in the annealing furnace is also caused due to configuration of the metal strip, such as bending, transverse curve and so forth. If the configuration of the metal strip is not suitable for passing through the annealing furnace to cause uneven contact with the hearth roll surface, meandering of the metal strip is caused even when the crown magnitude is sufficient. This tendency increases with increase of line speed. In order to regulate or adjust the configuration of the metal strip, tension leveler devices are provided.
In general, such tension leveler devices exert substantial tension, e.g. 20 kg/mm.sup.2. Such high tension exerted on the metal strip tends to cause breakage at the welded joint between leading and trailing metal coils, producing defects on the metal strip. On the other hand, because of the uneveness at the welded joint, leveling rolls in the tension leveler tend to be damaged due to high tension. In order to prevent breakage of the metal strip and/or damaging of the leveling rolls, steps have been taken to slow-down the line speed at the welded joint or to release the roll at the welded joint. Slow-down of the line speed necessarily causes lowering of the efficiency of the annealing furnace. On the other hand, releasing of the leveling rolls necessarily feeds the metal strip with unleveled portions to cause meandering in the annealing furnace.